Process for refining sands



March 19, 1968 PROCESS FOR REFINING SANDS Filed Oct. 10, 1966 HOTSAND f'l'G. 2

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United States Patent 3,374,062 PROSESd FUR REFKNHNG SANDS Frank W.Bowdish, 2440 11th St., Sparks, Nev. 89431 Qontinuation-in-part ofapplication Ser. No. 296,466,

July 22, 1963. This application Oct. 10, W66, Ser.

16 Claims. (Cl. 23-482) ABSTRACT OF THE DISCLQSURE The invention isaddressed to the refinement of sand having more than 0.5% by weight ironand a substantial amount of feldspar by the combination of steps whichincludes a first magnetic separation for splitting the eflluent from theseparator into one portion having a high iron content which is discardedand a product portion in which the iron content is less than 0.15% byweight. The product portion is heated to an elevated temperature andloaded into a leaching tank to form a column therein. A leachingsolution formed of an aqueous solution of sulphuric acid and alkalimetal chloride is preheated to a temperature within the range of 180 to225 F. and introduced into the tank from the bottom side at a ratesufi'iciently slow to enable the leaching solution to rise through thecolumn of hot sand whereby the first portion introduced will remain atthe top of the column whereby the portion of the leaching solution atthe top will contain the major portion of the iron dissolved from thesand so that removal of an increment of the upper portion of the columnof leaching solution will effect removal of the major portion of theiron content, the leaching solution is then allowed to drain downwardlyfrom the tank for re-use in subsequent leaching operations while thecolumn of sand is cleansed with wash water the initial portions of whichare added to form a part of the leaching solution.

This is a continuation-in-part of my copending application Ser. No.296,466, filed July 22, 1963, and entitled, Process for Refining Sand.

This invention relates to a process for refining sand and it relatesmore particularly to the refinement of sand for use in the preparationof glass and ceramics wherein the presence of feldspar is a desirablecost factor but in which the presence of iron is undesirable because ofits effect in discoloring the glass. Thus this invention is addressedmore particularly to the refinement of sand containing substantialamounts of feldspar for the removal of iron which may be present in theform of iron particles, magnetite, ilmenite, hematite and the like.Natural sands containing quartz and feldspar also usually contain someiron or iron-bearing materials which make the sand unsuitable for use inglass or ceramics without treatment to remove iron-bearing componentsbut without corresponding removal of quartz or feldspar or otherdesirable components.

Thus it is an object of this invention to provide a method and means forthe treatment of sand to effect substantial reduction in iron content to'a level below that which will have any undesirable effect on the use ofthe sand in the manufacture of glass or ceramics. More particularly, itis an object of this invention to provide a manufacturing process forthe refinement of sand to effect removal of iron without correspondingremoval of other of the desirable minerals present in the sand, such asquartz, feldspar and the like; in which the process can be carried outin a simple and efiicient manner, in

which the process can be carried out in large scale operations, usingcommercially available equipment and a minimum amount of labor andmaterials; and in which a high yield of low iron content sand isproduced at minimum cost.

These and other objects of this invention will hereinafter appear andfor purposes of illustration, but not of limitation, an embodiment ofthe invention is shown in the accompanying drawing, in which- FIG. 1 isa schematic view in section of the arrangement of elements for magneticseparation, and

FIG. 2 is a diagrammatic sketch of equipment arranged to show the flowof materials for effecting the leaching operation.

Some natural sand deposits consist of fragments of the more resistantminerals contained in. the parent rock from which the sand was derived.From such deposits much of the softer and weathered material such asclay and mud has either been washed away by natural streams or may beremoved by washing to leave sand particles that are sound. Such materialis often washed and sized to produce sand for construction and otherpurposes. Fine sand from such operations is generally either discardedor used for fill material, in asphalt paving mixtures, or for otherpurposes where it has a very low uiiit value. In locations where thecomposition of the sand is suitable, either the fine sand usuallyrejected from the construction sand or some coarser fractions of sandcan be treated by the novel process disclosed herein to yield a mixtureof minerals than can be substituted for at least part of the mineralconcentrates otherwise required in the manufacture of ceramics andglass.

Rocks containing feldspar or minerals that may be sub stituted forfeldspar and/or quartz together with other minerals may also be treatedby the process disclosed herein by first crushing and grinding the rocksto produce an artificial sand for treatment by the process. Unlessotherwise specified, the word sand as used hereinafter shall beunderstood to mean a particulate material whether is occurs naturally oris obtained by comminution of coarser sand or gravel or solid rock.

The sands to which this invention may be applied commonly containsubstantial fractions of quartz and/or feldspar minerals together withminor quantities of other weathering and abrasion resistant minerals.These minor minerals may include iron-bearing minerals such asmagnetite, ilmenite, biotite, hornblendes, and other accessory mineralscontained in the rocks from which the sand was derived. Magneticseparation, gravity separation, or flotation may often be used to removepart of the iron content of the sand in the form of these iron-bearingminerals. Only in exceptional cases, however, will such treatment aloneproduce a feldspar-quartz product sufficiently low in iron to permit itssubstitution for the mineral concentrates normally used in glass andceramics.

This invention will hereinafter be described with reference to therefinement of sand which contains substantial amounts, such as up to 25%by weight or more of feldspar. The latter is a desirable component incompositions used to make glass and ceramics and it constitutes anexpensive raw material. Thus sands containing substantial amounts offeldspar have sufficient-value to make is economically feasible toinvest substantially in the refinement of the sand to effect the removalof undesirable ingredients and it is desirable'to effect such refinementfor the removal of iron which is usually present in sands containingsubstantial amounts of feldspar.

While the invention has particular application to the refinement of sandhaving a high feldspar content, it will be understood that theconceptsof the invention may be employed in the treatment of sand forthe removal of iron where feldspar is not present or present only inminor amounts.

Experiments with many quartz-feldspar sands have shown that the mixedmineral product obtained after the most intense magnetic separation,gravity separation, or flotation of iron-bearing materials, stillcontains too much iron for use in glass or ceramic manufacture. Feldsparconcentrates made from such sands by flotation also contain excessiveamounts of iron, only part of which can be removed by magnetic, gravity,flotation, or scrubbing procedures. Other experiments have shown thatprolonged treatment with hot acids is also incapable of producing asuitable low-iron product for manufacture of clear substantiallycolorless glass.

The process disclosed herein resulted from a study of the reasons forthe failure of the experiments mentioned above. Iron occurs in severalforms in the sand mixtures of minerals to which this invention may beapplied. A substantial part, often a major part, of the iron in suchsands occurs as grains of iron-bearing minerals which are highlyresistant to dissolution in acids. For this reason acid treatment aloneof the sands will not lower their iron content SllffiClBllilY to makethem usable for glass, ceramic, or chemical manufacturing.

Iron also occurs as a part of the sand particles remaining behind afterflotation, magnetic separation, or gravity separation of theiron-bearing minerals. At least part of the'iron that may not be removedmagnetically, by gravity or by flotation must be on the surface of theparticles because abrasive scrubbing will remove part of it. This isespecially true of natural sands that have had their surfaces exposed toweathering and iron-bearing solutions .for a very long time. However,even abrasive scrubbing used in combination with the other mechanicalprocesses mentioned will not usually result in sufficient iron removal.Very long exposure of natural sand particles, and especially-particlesof feldspar, to the weathering influence of air and water with naturallydissolved salts seems tohave created cracks in or pits on the surface ofthe mineral particles within which iron compounds have been deposited.Small amounts of such iron impregnation in a mineral particle will notmake it respond to any of the methods for removing iron-bearing mineralparticles, nor can it be removed by abrasive scubbing of the surface ofthe particle.

The invention will now be described with reference to the procedures andequipment for the refinement, using a waste sand discarded as unsuitablefor use in the mixing of concrete but which can have considerable valueas a glass sand because of its high content of feldspar (about 25% byweight) but which is currently unsuitable for use as glass sand becauseof its relatively high iron content, often higher than 1% by weight ofiron. It will be understood that other sources of sand can be used, suchas river bottom sand, coarse sand, dune sand and the like.

For use in the practice of this invention it is preferred to make use ofa fine sand such as a sand having a mesh size passing through a 28 Tylerscreen or US-30. In the event that raw material constitutes a coarsesand, it is desirable to grind the sand to the finer mesh size desiredfor use in the practice of this invention.

Exam'ple 1.Magnetic separation Referring to FIG. 1 of the drawing, a drysand is cascaded over the periphery of a rotating magnetic roll 12located between pole pieces 14 and 16 connected to a suitable electricalsource. The sand 10 falls from the periphery of the magnetic roll with aspread or angle depending upon the magnetic attraction between the sandparticles and the magnetic roller whereby the sand particles most highlyattracted by reason of the presence of magnetic iron are displacedinwardly from the normal path of travel of the non-magnetic materialwhich is relatively unattracted by the magnetic roller.

A splitter 18 is adjustably located in the path of the sand particlesfalling from the roller to split the stream into an inner portion whichis formed of the sand particles containing the more magnetic componentsand an outer portion formed of the sand particles having little, if any,magnetic attraction for the roller. Thus the splitter 18 can be set tocut the stream at any desirable fraction. The more highly magneticportion cut from the main stream by the splitter is usually discarded asunsuitable for subsequent treatment.

It has been found that it is desirable to effect removal not only of thestrongly magnetic sand particles but also of the slightly magneticparticles in order to provide a product suitable for the subsequentleaching of the sand to remove iron in amounts suflicient to reduce theiron to a level low enough for use as a glass or ceramic sand.

When the pole piece 14 is spaced a substantial distance from theperiphery of the magnetic roller 12 to provide a substantial gap 20therebetween, 30-50% of the sand can be split from the stream to leaveonly 5070% of the sand for the subsequent leaching. Even this relativelysmall percentage of sand is incapable of yielding a product having asufficiently low iron content because of the excessive amounts of ironwhich still remain in the sand after magnetic separation. Splitting toremove a still higher percentage of sand by magnetic separation to yielda product having a correspondin ly lower iron content for use in thesubsequent leach is undesirable from the economical standpoint becauseof the large amount of material required to be processed to secure arelatively low amount of sand having a sufficiently low iron contentwhen processed in this manner.

It has been found that by modification to achieve a higher intensityfield in the magnetic separation such as by spacing the pole piece 14more closely to the periphery of the magnetic roller to reduce the gapthrough which the stream of sand travels, it is possible to effect dryseparation in air with the splitter set to remove only 25% of the flowfor discard while yielding a product suitable for further refinement bythe chemical leach. It has been found that an iron content of 0.15% byweight is the maximum that can be permitted in the feed from themagnetic separator to the chemical leach for removal of suflicient ironby the subsequent leach step to enable use of the sand in themanufacture of glass or ceramics. The sand split from the stream issuingfrom the periphery of the magnetic separator contains iron in forms andin locations which often cannot be leached by the chemical leach, suchas inclusions within the crystal structure.

By way of illustration, but not by way of limitation, the desiredseparation to yield a sand having less than 0.15 by weight of iron canbe effected with a magnetic roller having the'pole piece 14 spaced about/s inch from the periphery of the roller with the coil of the magnethaving a current density suflicient to develop a magnetic circuit thatis fully saturated with 90,000 to 120,000 lines of force per square inchacross the space between the pole pieces and the periphery of theroller. Under these conditions, feed having an original iron content of1% or more yields a product which averages about 0.13 to 0.15% by weightof iron in which the product constitutes by weight of the original feed.

In practice, use has been made of a five-high roll magnetic'separator inwhich the pole pieces are set at different distances from the peripheryof the roller with the maximum gap at the top and with the minimum gapat the bottom with the sand particles cascaded over the rollers with theunsplit portion from the upper roll constituting the feed to the nextlower roll. By this arrangement, the iron and magnetic material will beremoved mostly by the top roller whereby the feed to the lower rollerswill constitute the lesser magnetic materials in the form of lockedparticles of magnetic material such as hematite with little, if any,magnetite.

Example 2.-Chemical [each The non-magnetic sand particles from themagnetic separator are next subjected to a chemical leach with an acidicsolution at elevated temperature.

For this purpose, the sand is preheated to elevated temperature,preferably to a temperature Within the range of 180 to 225 F. and morepreferably to a temperature within the range of 200 to 225 F. While itis possible to carry out the chemical leach without preheating of thesand, the time required to effect removal of the desired amount of ironmay take many months in the absence of leaching at elevated temperaturesuch that leaching without preheat would be impractical. Preheating to atemperature below 180 F. will reduce the time required for leaching butthe circulation of excessive amounts of hot chemical leach solutionwould be required to achieve the temperature conditions desired foroptimum results from the leaching operation. Heating of the sand to atemperature in excess of 225 F. or above the boiling point of theleaching solution will result in undesirable boiling of the solutionupon contact with the sand and the loss of volatiles to the atmospherewhereby an unstable process and unhealthy atmosphere may result.

While the temperature of the sand can be raised to the desired level bythe chemical leaching solution when heated to a temperature higher thanthat desired for the leaching operation, the time required for raisingthe sand to the desired temperature would be increased and an unstablesystem would result from the excessive heating of the leaching solution.In the preferred practice of this invention, the sand is preheated in arotary kiln preferably to a temperature sufficiently above thetemperature desired for the chemical leaching operation to compensatefor heat losses incurred during transfer of the sand to the leach tankso that, when the sand is loaded into the tank, the mass will be at atemperature within the range of 180 to 225 F. Thus the sand may beheated in the kiln to a temperature up to about 260 F.

The hot sand is leached with an acidic solution containing an alkalimetal halide such as a sulphuric acid solution containing sodiumchloride, potassium chloride and the like alkali metal chloridedissolved therein. Any halide of a metal which would not cause sulfateprecipitation can be used, such as ammonium chloride, magnesium chlorideand the like. I

In the preferred practice of this invention, the aqueous leachingsolution is formulated to contain -20% by Weight of .H SO and 315%byweight of an alkali metal chloride, and preferably 10-15% by weight H80 and 610%' by weight alkali metal chloride. While use can be made ofan acid such as sulphuric acid, in the absence of the metal halide, thepresence of the latter in solution operates effectively to acceleratethe rate of leaching two to five fold whereby inclusion of the metalhalide in the chemicalleaching solution is. most desirableJWhile the useof an acid such as hydrochloric acid alone may re sult in rapidleaching, the reagent cost is several times as great and the highvolatility of hydrogen chloride in such solutions makes the use ofhydrochloric acid undesirable. The combination of sulphuric acid andsodium chloride results in rapid leaching, low reagent costs, and arelatively low level of obnoxious fumes.

In practice, the chemical leach makes use of the system illustrated inFIG. 2 in which the hot sand 10 is loaded into a leaching tank 40 whichis provided with a cover 42 at the top and with an inlet 44 at thebottom through which the leaching solution flows into and out of thetank and a larger outlet 46 at the bottom through which the refined sandis flushed from the tank. Spaced a short distance from the bottom of thetank is a filter plate or fabric 48 formed of a material such aspolypropylene which is resistant to the acidic leaching solution. Thetank walls are preferably lined with a material to protect the metalwalls from corrosion by the solution, such as a lining of Hypalonrubber, marketed by E. I. du Pont de Nemours & Company.

A leach solution 50 formulated of parts by weight water, 18 parts byweight concentrated sulphuric acid and 10 parts by weight sodiumchloride is stored in a separate storage vessel 52. The latter isconnected with the inlet 44 through a line 54 having a pump 56 for thedisplacement of liquid from the storage vessel 52 into the tank 40. Theline is also provided with a heat exchanger 58 through which the streamflows for heat exchange relation with a heating fluid to raise thetemperature of the aqueous leaching solution to leaching temperaturewithin the range of to 225 F. before introduction into the leaching tank40. The heat exchanger 58 can be of conventional construction, such as ashell having a plurality of tubular elements extending therethrough withthe heat exchange fluid constituting low pressure steam at about 240 to250 F.

The leaching solution, preferably heated to a temperature of 200 to 225F., is injected into the tank through the inlet 44 into the area beneaththe filter plate 48 so that it will rise gradually through the column ofsand loaded Within the tank to wet the sand particles and effect removalof soluble iron therefrom.

The leaching solution is injected into the leaching tank at a slow rateto permit unidirectional flow with a minimum of intermixing within thetank so that the liquid which first enters the tank will remain at thetop of the column of liquid introduced into the tank While the liquidlast to be introduced will remain at the bottom.

It has been determined that the major portion (50% or more) of the ironremaining in the sand loaded into the tank will be removed by theleaching solution within the first 5-10 minutes of contact. Thus, forexample, if the leaching solution is introduced at a rate to overflowthe column of sand in 100 minutes, then all of the sand will have beenin contact with the first A of the leaching solution introduced into thetank for at least 10 minutes. Thus some 50% of the residual iron on thesand will find its way into the solution forming the upper to of thecolumn of liquid in the leaching tank.

The introduction of leaching solution is continued at the same slow ratein amounts to overflow the top of the tank with the upper of the columnof solution which contains the majority of the iron leached from thesand. This first increment that is overflowed will carry most of theiron remaining in the sand to waste. In the preferred practice,introduction of leaching solution. is continued to overflow a secondincrement, such as the of the column immediately following the upperincrement discarded to waste. This second increment containing greaterproportions of iron than the remainder of the solution in the tank isoverflowed through line 60 for collection in an intermediate storagetank 62 for use as the initial increment of leaching solution introducedinto the tank to constitute the upper of the column in the next cycle ofleaching operation.

After the first increment of leaching solution has been overflowed towaste and the second increment overflowed to storage, the remainder ofthe leaching solution is retained at rest in the leaching tank for anadditional soaking time, such as 5-15 or more hours. In the illustratedmodification, the tank is 8 feet in diameter and has a capacity of about3000 gallons of liquid when filled with sand. Thus the first incrementwhich is overflowed to waste will amount to about 300 gallons ofleaching solution while the next 300 gallons will be overflowed to theintermediate storage 62. The large amount of material filling the tankenables the tank to hold the heat for a substantial period of time toachieve-the desired soak and leaching action during the holding period.

Removal of the leaching solution is effected by opening slightly valves64 and 22 to enable the solution to drain slowly by gravity from thebottom of the tank. As the solution is drawn down in the tank forreturnto the storage facility 52, wash solution is drawn from tank 24 andpumped by pump 56 through lines 54 and 66 onto the top of the sandcolumn at substantially the same rate as the leach solution is withdrawnthereby keeping the sand column covered at all times. After the washsolution from tank 24 has soaked into the top of the sand columnfollowing the leach solution as it drains down, but without permittingany air to be entrapped within the sand bed, a large quantity of washwater from inlet 26 is added to keep the sand covered with water whilethe leaching solution is drained rapidly into storage tank 52. Eventhrough valves 64 and 22 are opened fully for this drainage, very littleintermixing occurs between the layers of solution in the sand column,and about 90% of the leaching solution may be recovered without anydilution. The layer of wash solution from tank 24 following the top ofthe leach solution down through the sand column becomes enriched in acidand salt and dissolved iron washed from the sand, and most of thisenriched wash solution is also drained into tank 52 in order that theacid and salt it contains may be reused, and further to provide waterfor making up the leaching solution to its original volume andcomposition for the next leaching cycle. The first increment of washwater overlying the wash solution layer is drained through valve 28 intotank 24 by closing valve 22, and the remainder of the wash water isdrained to waste through valve 30. The first increment of wash waterwill contain most of the leaching solution left entrained on theparticles of sand after passage of the wash solution thereby to enablerecovery of most of the leaching solution. Any acid or salt remaining inthe column of sand will be washed from the sand by the additionalincrements of wash water flowing downwardly therethrough.

After the leach solution has been drained from the tank and the sandwashed with water to remove residual acids, the main valve 70 is openedand water is introduced through valve 32 into the tank in relativelyhigh volume beneath the filter whereby the water flows upwardly throughthe filter into engagement with the leached sand to wash the sand fromthe tank through the passage 72. The sand which is washed from the tankis led to a screw washer 74 which drains and lifts the solids particlesof sand to a bin 76 for further drainage after which the sand is driedin a conventional kiln drier 78 to yield a product having about 0.03% byweight residual iron thereby to provide a product which is well adaptedfor use in the production ofglass and ceramics.

The foregoing process is well adapted for use in the treatment of sandhaving substantial amounts of limestone or calcium carbonate whichdissolves in the acidic leaching solution during removal of the residualiron. It has been found that the dissolved calcium slowly forms intocalcium sulphate which precipitates probably in the form of gypsum. Inthe absence of the removal of the first increment of solution whichcomes into contact with the sand, as previously described, the formedcalcium sulphate crystals interefere with the free flow of liquid intoand out of the tank and they also block the pipe lines and foul the heatexchanger tubes. When, as described, the leaching solution is caused torise slowly through the column of sand, the calcium, which is dissolvedin the first increment, will be carried from the system with the firstincrement of liquid that rises to the top and is discarded.

As used herein and in the claims, the term iron is intended to includenot only elemental iron but iron in its various combined forms.

It will be apparent from the foregoing that I have provided an efiicientmass production process which is capable of utilization of valuablesands containing substantial amounts of feldspar to effect removal ofingredients which are undesirable in the use of the sand in thefabrication of glass, ceramics, and other substantially colorlessproducts.

It will be understood that changes may be made in the details offormulationand methods and conditions of operation without departingfrom the spirit of the invention especially as defined in the followingclaims.

I claim:

1. A process for refining sandfor the removal of iron comprising thesteps of passing the sand in finely'divided form through a magneticseparator, splitting the effluent from the separator between a portionhaving a high iron content which is discarded and a product portionhaving an iron content of not more than 0.15% by weight, heat treatingthe product portion to an elevated temperature, loading the hot sandinto a leaching tank to form a column of sand therein, preheating achemical leaching solution and introducing the leaching solution heatedto a temperature not to exceed 225 F. into the bottom portion of thetank at a rate suificiently slow to cause the leaching solution to risethrough the column of-sand in the tank whereby the first portionintroduced will remain at the top of the column of leaching solutionrising through the column of sand in-the tank whereby a major portion ofthe iron remainingin the sand after magnetic separation will be removedin the portion of the liquid at the top of the column, removing theincrement of leaching solution at the top of the column forcarrying offthe dissolved iron and other soluble impuritieswhile continuing tointroduce an equivalent amount-at the bottom to fill the tank withleaching solution at least to the level of the sand in the tank, holdingthe column 'of leaching solution in contact with the sand for anadditional soaking time, draining the leaching solution from the tank,adding an aqueous wash at the top at a rate corresponding to the rate ofdrainage'of the leaching solution from the tank to keep the column ofsand covered while in contact with the leaching solutionto prevententrapment of air and then flushing the leached sand from the tank forremoval as product.

2. The process as claimed in claim 1 in which the-sand subjected to themagnetic separation step has a mesh size of less than 28 Tyler screen,

3. The process as claimed in claim 1 in which the-sand subjected torefinement has more than 0.5% by weight of iron and a substantial amountof feldspar.

4. The process as claimed in claim 1 in which the magnetic separation iseffected by cascading the sand as a dry stream over a rotating magneticroller and through a gap of to 7& inch between the periphery of'theroller and the adjacent magnetic pole piece.

5. The process as claimed in claim 1 in which .the preheated sand is ata temperature within the range of 180 to 225 F. in the leaching tank.

6. The process as claimed in claim 1 in which the chemical leachingsolution comprises an aqueous solution of sulphuric acid and an alkalimetal chloride.

7. The process as claimed in claim.6 in which the aqueous leachingsolution contains 10-20% by weight H and 3-15 by weight alkali'metalchloride.

8. The process as claimed in claim 6 in which the alkali metal chlorideis sodium chloride and in which the materials are present in the aqueoussolutionin an amount within the range of 610% by weight H 50 and 8-12%by weight sodium chloride.

'9. The process as claimed in claim 1 in which the leaching reaction iscarried out with the materials-at a temperature within the range of to225 F.

10. The process as claimed in claim 1 in which the leaching solutionis-introduced at a rate corresponding to less than A of the volume ofthe tankper minute;

11. The process as claimed in claim 10 in which the amount of leachingsolution removed from the top-of the column comprises at leastthe upperA of the column to carry off the major portion of the iron-dissolvedoutof the sand,

12. The process as claimed in claim 1 in Which the leaching solution isremoved from the top of the column y tion i solution into the bottomportion of the tank to overflow the upper portion of the column from thetop of the tank.

13- The process as claimed in claim 1; in which the ieached sand isflushed from the tank by introducing water through the bottom portion ofthe tank for engagement with the underside of the column of sand Withinthe tank.

14 The process as claimed in claim 1. which includes the additional stepof drying the sand flushed from the leaching tank.

15. The process as claimed in claim 1 incdules the step of removing asecond increment of leaching solution immediately below that portion ofthe column of leaching solution removed from the system and introducingsaid second increment as the first portion of leaching solutionintroduced into the tank in the next cycle or" operation.

16. The process as claimed in claim 1 which includes the step ofremoving an increment of wash water immediately above the column ofleach and wash solutions and introducing said removed increment as theWash solu- It) tion ahead of the Wash Water in the next cycle ofoperation.

References Cited UNITED STATES PATENTS 889,129 5/1908 Hendryx 7S-1101,957,579 5/1934 Crew 23182 2,182,384 12/1939 McGregor 252-8 2,306,02112/1942 Knowles et a1 23l82 FOREIGN PATENTS 482,243 3/1937 GreatBritain.

OTHER REFERENCES Perry (Editor), Chemical Engineers Handbook, thirdedition (1950), pp. 1092 and 1093 relied on.

EDWARD J. MEROS, Primary Examiner. OSCAR R. VERTIZ, Examiner.

A. GREIF, Assistant Examiner.

